Nebula

nebula,  (Latin: “mist” or “cloud”)plural nebulae or nebulas, any of the various tenuous clouds of gas and dust that occur in interstellar space. The term was formerly applied to any object outside the solar system that had a diffuse appearance rather than a pointlike image, as in the case of a star. This definition, adopted at a time when very distant objects could not be resolved into great detail, unfortunately includes two unrelated classes of objects: the extragalactic nebulae, now called galaxies, which are enormous collections of stars and gas, and the galactic nebulae, which are composed of the interstellar medium (the gas between the stars, with its accompanying small solid particles) within a single galaxy. Today the term nebula generally refers exclusively to the interstellar medium.
In a spiral galaxy the interstellar medium makes up 3 to 5 percent of the galaxy’s mass, but within a spiral arm its mass fraction increases to about 20 percent. About 1 percent of the mass of the interstellar medium is in the form of “dust”—small solid particles that are efficient in absorbing and scattering radiation. Much of the rest of the mass within a galaxy is concentrated in visible stars, but there is also some form of dark matter that accounts for a substantial fraction of the mass in the outer regions.
The most conspicuous property of interstellar gas is its clumpy distribution on all size scales observed, from the size of the entire Milky Way Galaxy (about 10²⁰ metres, or hundreds of thousands of light-years) down to the distance from Earth to the Sun (about 10¹¹ metres, or a few light-minutes). The large-scale variations are seen by direct observation; the small-scale variations are observed by fluctuations in the intensity of radio waves, similar to the “twinkling” of starlight caused by unsteadiness in the Earth’s atmosphere. Various regions exhibit an enormous range of densities and temperatures. Within the Galaxy’s spiral arms about half the mass of the interstellar medium is concentrated in molecular clouds, in which hydrogen occurs in molecular form (H₂) and temperatures are as low as 10 kelvins (K). These clouds are inconspicuous optically and are detected principally by their carbon monoxide (CO) emissions in the millimetre wavelength range. Their densities in the regions studied by CO emissions are typically 1,000 H₂ molecules per cubic cm. At the other extreme is the gas between the clouds, with a temperature of 10 million K and a density of only 0.001 H⁺ ion per cubic cm. Such gas is produced by supernovae, the violent explosions of unstable stars.
This article surveys the basic varieties of galactic nebulae distinguished by astronomers and their chemical composition and physical properties.

Classes of nebulae

All nebulae observed in the Milky Way Galaxy are forms of interstellar matter—namely, the gas between the stars that is almost always accompanied by solid grains of cosmic dust. Their appearance differs widely, depending not only on the temperature and density of the material observed but also on how the material is spatially situated with respect to the observer. Their chemical composition, however, is fairly uniform; it corresponds to the composition of the universe in general in that approximately 90 percent of the constituent atoms are hydrogen and nearly all the rest are helium, with oxygen, carbon, neon, nitrogen, and the other elements together making up about two atoms per thousand. On the basis of appearance, nebulae can be divided into two broad classes: dark nebulae and bright nebulae. Dark nebulae appear as irregularly shaped black patches in the sky and blot out the light of the stars that lie beyond them. Bright nebulae appear as faintly luminous glowing surfaces; they either emit their own light or reflect the light of nearby stars.
Dark nebulae are very dense and cold molecular clouds; they contain about half of all interstellar material. Typical densities range from hundreds to millions (or more) of hydrogen molecules per cubic centimetre. These clouds are the sites where new stars are formed through the gravitational collapse of some of their parts. Most of the remaining gas is in the diffuse interstellar medium, relatively inconspicuous because of its very low density (about 0.1 hydrogen atom per cubic cm) but detectable by its radio emission of the 21-cm line of neutral hydrogen.
Bright nebulae are comparatively dense clouds of gas within the diffuse interstellar medium. They have several subclasses: (1) reflection nebulae, (2) H II regions, (3) diffuse ionized gas, (4) planetary nebulae, and (5) supernova remnants.
Reflection nebulae reflect the light of a nearby star from their constituent dust grains. The gas of reflection nebulae is cold, and such objects would be seen as dark nebulae if it were not for the nearby light source.
H II regions are clouds of hydrogen ionized (separated into positive H⁺ ions and free electrons) by a neighbouring hot star. The star must be of stellar type O or B, the most massive and hottest of normal stars in the Galaxy, in order to produce enough of the radiation required to ionize the hydrogen.
Diffuse ionized gas, so pervasive among the nebular clouds, is a major component of the Galaxy. It is observed by faint emissions of positive hydrogen, nitrogen, and sulfur ions (H⁺, N⁺, and S⁺) detectable in all directions. These emissions collectively require far more power than the much more spectacular H II regions, planetary nebulae, or supernova remnants that occupy a tiny fraction of the volume.
Planetary nebulae are ejected from stars that are dying but are not massive enough to become supernovae—namely, red giant stars. That is to say, a red giant has shed its outer envelope in a less-violent event than a supernova explosion and has become an intensely hot star surrounded by a shell of material that is expanding at a speed of tens of kilometres per second. Planetary nebulae typically appear as rather round objects of relatively high surface brightness. Their name is derived from their superficial resemblance to planets—i.e., their regular appearance when viewed telescopically as compared with the chaotic forms of other types of nebula.
Supernova remnants are the clouds of gas expanding at speeds of hundreds or even thousands of kilometres per second from comparatively recent explosions of massive stars. If a supernova remnant is younger than a few thousand years, it may be assumed that the gas in the nebula was mostly ejected by the exploded star. Otherwise, the nebula would consist chiefly of interstellar gas that has been swept up by the expanding remnant of older objects.

Historical survey of the study of nebulae

Pre-20th-century observations of nebulae

In 1610, two years after the invention of the telescope, the Orion Nebula, which looks like a star to the naked eye, was discovered by the French scholar and naturalist Nicolas-Claude Fabri de Peiresc. In 1656 Christiaan Huygens, the Dutch scholar and scientist, using his own greatly superior instruments, was the first to describe the bright inner region of the nebula and to determine that its inner star is not single but a compact quadruple system.
Early 18th-century observational astronomers gave high priority to comet seeking. A by-product of their search was the discovery of many bright nebulae. Several catalogs of special objects were compiled by comet researchers; by far the best known is that of the Frenchman Charles Messier, who in 1781 compiled a catalog of 103 nebulous, or extended, objects in order to prevent their confusion with comets. Most are clusters of stars, 35 are galaxies, and 11 are nebulae. Even today many of these objects are commonly referred to by their Messier catalog number; M20, for instance, is the great Trifid Nebula, in the constellation Sagittarius.

The work of the Herschels

By far the greatest observers of the early and middle 19th century were the English astronomers William Herschel and his son John. Between 1786 and 1802 William Herschel, aided by his sister Caroline, compiled three catalogs totaling about 2,500 clusters, nebulae, and galaxies. John Herschel later added to the catalogs 1,700 other nebulous objects in the southern sky visible from the Cape Observatory in South Africa but not from London and 500 more objects in the northern sky visible from England.
The catalogs of the Herschels formed the basis for the great New General Catalogue (NGC) of J.L. Dreyer, published in 1888. It contains the location and a brief description of 7,840 nebulae, galaxies, and clusters. In 1895 and 1908 it was supplemented by two Index Catalogues (IC) of 5,386 additional objects. The list still included galaxies as well as true nebulae, for they were often at this time still indistinguishable. Most of the brighter galaxies are still identified by their NGC or IC numbers according to their listing in the New General Catalogue or Index Catalogues.

Advances brought by photography and spectroscopy

The advent of photography, which allows the recording of faint details invisible to the naked eye and provides a permanent record of the observation for study of fine details at leisure, caused a revolution in the understanding of nebulae. In 1880 the first photograph of the Orion Nebula was made, but really good ones were not obtained until 1883. These early photographs showed a wealth of detail extending out to distances unsuspected by visual observers.
Much can be learned about the physical nature of an astronomical object by studying its spectrum—i.e., the resolution of its light into different wavelengths (or colours). Study of the spectrum of an object provides a decisive test as to whether it is composed of unresolved stars (as are galaxies) or glowing gas. Stars radiate at all wavelengths, almost always with dark absorption lines superimposed, while hot, transparent gas clouds radiate only emission lines at certain wavelengths characteristic of their constituent gases. In 1864 observation of the spectrum of the Orion Nebula showed bright emission lines of glowing gases, with conspicuous hydrogen lines and some green lines even brighter. By contrast, the spectrum of galaxies was found to be stellar, so a distinction between galaxies and nebulae—that nebulae are gaseous and galaxies are stellar—was appreciated at this time, although the true sizes and distances of galaxies were not demonstrated until the 20th century.

20th-century discoveries

The 20th century witnessed enormous advances in observational techniques as well as in the scientific understanding of the physical processes that operate in interstellar matter. In 1930 a German optical worker, Bernhard Schmidt, invented an extremely fast wide-angled camera ideal for photographing faint extended nebulae. Photographic plates became progressively more sensitive to an ever-widening range of colours, but photography has been completely replaced by photoelectric devices. Most images are now recorded with so-called charge-coupled devices (CCDs) that act as arrays of tiny photoelectric cells, each recording the light from a small patch of sky. Modern CCDs consist of square arrays of up to 4,000 cells on each side, or 16 million independent photocells, capable of observing the sky simultaneously. Electronic detectors are up to 100 times more sensitive than photography, can record a much wider range of light levels, and are sensitive to a much wider range of wavelengths, from 0.1 micrometre in the ultraviolet (accessible only from satellites orbiting above Earth’s atmosphere) to more than 1.2 micrometres in the infrared.
Spacecraft allow the observation of radiation normally absorbed by Earth’s atmosphere: gamma and X-rays (which have very short wavelengths), far-ultraviolet radiation (with wavelengths shorter than about 0.3 micrometre, below which atmospheric ozone is strongly absorbing), and infrared (from about 3 micrometres to 1 mm), strongly absorbed by atmospheric water vapour and carbon dioxide. Gamma rays, X-rays, and ultraviolet radiation reveal the physical conditions in the hottest regions in space (extending to some 100 million kelvins in shocked supernova gas). Infrared radiation reveals the conditions within dark cold molecular clouds, into which starlight cannot penetrate because of absorbing dust layers.
The primary means of studying nebulae is not by images but by spectra, which show the relative distribution of the radiation among various wavelengths (or colours for optical radiation). Spectra can be obtained by means of prisms (as in the earlier part of the 20th century), diffraction gratings, or crystals, in the case of X-rays. A particularly useful instrument is the echelle spectrograph, in which one coarsely ruled grating spreads the electromagnetic radiation in one direction, while another finely ruled grating disperses it in the perpendicular direction. This device, often used both in spacecraft and on the ground, allows astronomers to record simultaneously a wide range of wavelengths with very high spectral resolution (i.e., to distinguish slightly differing wavelengths). For even higher spectral resolution astronomers employ Fabry-Pérot interferometers. Spectra provide powerful diagnostics of the physical conditions within nebulae. Images and spectra provided by Earth-orbiting satellites, especially the Hubble Space Telescope, have yielded data of unprecedented quality.
Ground-based observations also have played a major role in recent advances in scientific understanding of nebulae. The emission of gas in the radio and submillimetre wavelength ranges provides crucial information regarding physical conditions and molecular composition. Large radio telescope arrays, in which several individual telescopes function collectively as a single enormous instrument, give spatial resolutions in the radio regime far superior to any yet achieved by optical means.

Chemical composition and physical processes

Many characteristics of nebulae are determined by the physical state of their constituent hydrogen, by far the most abundant element. For historical reasons, nebulae in which hydrogen is mainly ionized (H⁺) are called H II regions, or diffuse nebulae; those in which hydrogen is mainly neutral are designated H I regions; and those in which the gas is in molecular form (H₂) are referred to as molecular clouds. The distinction is important because of major differences in the radiation that is present in the various regions and consequently in the physical conditions and processes that are important. Radiation is a wave but is carried by packets called photons. Each photon has a specified wavelength and precise energy that it carries, with gamma rays (short wavelengths) carrying the most and X-rays, ultraviolet, optical, infrared, microwave, and radio waves following in order of decreasing energies (or increasing wavelengths). Neutral hydrogen atoms are extremely efficient at absorbing ionizing radiation—that is, an energy per photon of at least 13.6 electron volts (or, equivalently, a wavelength of less than 0.0912 micrometre). If the hydrogen is mainly neutral, no radiation with energy above this threshold can penetrate except for photons with energies in the X-ray range and above (thousands of electron volts or more), in which case the hydrogen becomes somewhat transparent. The absorption by neutral hydrogen abruptly reduces the radiation field to almost zero for energies above 13.6 electron volts. This dearth of hydrogen-ionizing radiation implies that no ions requiring more ionizing energy than hydrogen can be produced, and the ionic species of all elements are limited to the lower stages of ionization. Within H II regions, with almost all the hydrogen ionized and thereby rendered nonabsorbing, photons of all energies propagate, and ions requiring energetic radiation for their production (e.g., O⁺⁺) occur.
Ultraviolet photons with energies of more than 11.2 electron volts can dissociate molecular hydrogen (H₂) into two H atoms. In H I regions there are enough of these photons to prevent the amount of H₂ from becoming large, but the destruction of H₂ as fast as it forms takes its toll on the number of photons of suitable energies. Furthermore, interstellar dust is a fairly efficient absorber of photons throughout the optical and ultraviolet range. In some regions of space the number of photons with energies higher than 11.2 volts is reduced to the level where H₂ cannot be destroyed as fast as it is produced on grain surfaces. In this case, H₂ becomes the dominant form of hydrogen present. The gas is then part of a molecular cloud. The role of interstellar dust in this process is crucial because H₂ cannot be formed efficiently in the gas phase.

Interstellar dust

Only about 0.7 percent of the mass of the interstellar medium is in the form of solid grains, but these grains have a profound effect on the physical conditions within the gas. Their main effect is to absorb stellar radiation; for photons unable to ionize hydrogen and for wavelengths outside absorption lines or bands, the dust grains are much more opaque than the gas. The dust absorption increases with photon energy, so long-wavelength radiation (radio and far-infrared) can penetrate dust freely, near-infrared rather well, and ultraviolet relatively poorly. Dark, cold molecular clouds, within which all star formation takes place, owe their existence to dust. Besides absorbing starlight, the dust acts to heat the gas under some conditions (by ejecting electrons produced by the photoelectric effect, following the absorption of a stellar photon) and to cool the gas under other conditions (because the dust can radiate energy more efficiently than the gas and so in general is colder). The largest chemical effect of dust is to provide the only site of molecular hydrogen formation on grain surfaces. It also removes some heavy elements (especially iron and silicon) that would act as coolants to the gas. The optical appearance of most nebulae is significantly modified by the obscuring effects of the dust.
The chemical composition of the gas phase of the interstellar medium alone, without regard to the solid dust, can be determined from the strength of narrow absorption lines that are produced by the gas in the spectra of background stars. Comparison of the composition of the gas with cosmic (solar) abundances shows that almost all the iron, magnesium, and silicon, much of the carbon, and only some of the oxygen and nitrogen are contained in the dust. The absorption and scattering properties of the dust reveal that the solid grains are composed partially of silicaceous material similar to terrestrial rocks, though of an amorphous rather than crystalline variety. The grains also have a carbonaceous component. The carbon dust probably occurs in at least two forms: (1) grains, either free-flying or as components of composite grains that also contain silicates, and (2) individual, freely floating aromatic hydrocarbon molecules, with a range varying from 70 to several hundred carbon atoms and some hydrogen atoms that dangle from the outer edges of the molecule or are trapped in the middle of it. It is merely convention that these molecules are referred to as dust, since the smallest may be only somewhat larger than the largest molecules observed with a radio telescope. Both of the dust components are needed to explain spectroscopic features arising from the dust. In addition, there are probably mantles of hydrocarbon on the surfaces of the grains. The size of the grains ranges from perhaps as small as 0.0003 micrometre for the tiniest hydrocarbon molecules to a substantial fraction of a micrometre; there are many more small grains than large ones.
The dust cannot be formed directly from purely gaseous material at the low densities found even in comparatively dense interstellar clouds, which would be considered an excellent laboratory vacuum. For a solid to condense, the gas density must be high enough to allow a few atoms to collide and stick together long enough to radiate away their energy to cool and form a solid. Grains are known to form in the outer atmospheres of cool supergiant stars, where the gas density is comparatively high (perhaps 10⁹ times what it is in typical nebulae). The grains are then blown out of the stellar atmosphere by radiation pressure (the mechanical force of the light they absorb and scatter). Calculations indicate that refracting materials, such as the constituents of the grains proposed above, should condense in this way.
There is clear indication that the dust is heavily modified within the interstellar medium by interactions with itself and with the interstellar gas. The absorption and scattering properties of dust show that there are many more smaller grains in the diffuse interstellar medium than in dense clouds; apparently in the dense medium the small grains have coagulated into larger ones, thereby lowering the ability of the dust to absorb radiation with short wavelengths (namely, ultraviolet, near 0.1 micrometre). The gas-phase abundances of some elements, such as iron, magnesium, and nickel, also are much lower in the dense regions than in the diffuse gas, although even in the diffuse gas most of these elements are missing from the gas and are therefore condensed into dust. These systematic interactions of gas and dust show that dust grains collide with gas atoms much more rapidly than one would expect if the dust and gas simply drifted together. There must be disturbances, probably magnetic in nature, that keep the dust and gas moving with respect to each other.
The motions of gas within nebulae of all types are clearly chaotic and complicated. There are sometimes large-scale flows, such as when a hot star forms on the outer edge of a cold, quiescent dark molecular cloud and ionizes an H II region in its vicinity. The pressure strongly increases in the newly ionized zone, so the ionized gas flows out through the surrounding material. There are also expanding structures resembling bubbles surrounding stars that are ejecting their outer atmospheres into stellar winds.

Turbulence

Besides these organized flows, nebulae of all types always show chaotic motions called turbulence. This is a well-known phenomenon in gas dynamics that results when there is low viscosity in flowing fluids, so the motions become chaotic eddies that transfer kinetic and magnetic energy and momentum from large scales down to small sizes. On small-enough scales viscosity always becomes important, and the energy is converted into heat, which is kinetic energy on a molecular scale. Turbulence in nebulae has profound, but poorly understood, effects on their energy balance and pressure support.
Turbulence is observed by means of the widths of the emission or absorption lines in a nebular spectrum. No line can be precisely sharp in wavelength, because the energy levels of the atom or ion from which it arises are not precisely sharp. Actual lines are usually much broader than this intrinsic width because of the Doppler effect arising from motions of the atoms along the line of sight. The emission line of an atom is shifted to longer wavelengths if it is receding from the observer and to shorter wavelengths if it is approaching. Part of the observed broadening is easily explained by thermal motions, since v², the averaged squared speed, is proportional to T/m, where T is the temperature and m is the mass of the atom. Thus, hydrogen atoms move the fastest at any given temperature. Observations show that in fact hydrogen lines are broader than those of other elements but not as much as expected from thermal motions alone. Turbulence represents bulk motions, independent of the mass of the atoms. This chaotic motion of gas atoms of all masses would explain the observations. The physical question, though, is what maintains the turbulence. Why do the turbulent cascades not carry kinetic energy from large-size scales into ever-shorter-size scales and finally into heat?
The answer is that energy is continuously injected into the gases by a variety of processes. One involves strong stellar winds from hot stars, which are blown off at speeds of thousands of kilometres per second. Another arises from the violently expanding remnants of supernova explosions, which sometimes start at 20,000 km (12,000 miles) per second and gradually slow to typical cloud speeds (10 km [6 miles] per second). A third process is the occasional collision of clouds moving in the overall galactic gravitational potential. All these processes inject energy on large scales that can undergo turbulent cascading to heat.

Galactic magnetic field

There is a pervasive magnetic field that threads the spiral arms of the Milky Way Galaxy and extends to thousands of light-years above the galactic plane. The evidence for the existence of this field comes from radio synchrotron emission produced by very energetic electrons moving through it and from the polarization of starlight that is produced by elongated dust grains that tend to be aligned with the magnetic field. The magnetic field is very strongly coupled to the gas because it acts upon the embedded electrons, even the few in H I regions, and the electrons impart some motion of the other constituents by means of collisions. The gas and field are effectively confined to moving together, even though the gas can slip along the field freely. The field has an important influence upon the turbulence because it exerts a pressure similar to gas pressure, thereby influencing the motions of the gas. The resulting complex interactions and wave motions have been studied in extensive numerical calculations.